HALOCORDYLE COOPERI. 213 



(2) The oral verticil of four capitate tentacles tends to be 

 constant, except in the few cases where all the tentacles were 

 irregularly scattered. 



(3) The variations in symmetry show how easily H. 

 cooperi could have descended from a Pennarian ancestor, 

 where the capitate tentacles are present in a considei'able 

 number, and are quite irreguhirly scattered. 



(4) An important point to consider is whether the dis- 

 tribution of symmeti-y in a population throws any h"g-ht on 

 the steps by means of which this symmetry was acquired. 



It may be assumed that symmetry of this nature could not 

 easily be acquired by imperceptible steps, it would more 

 readily be acquired by larger steps, or in other words by 

 " discontinuous " variations. 



On account of the tendency which the observations have 

 to form a symmetrical curve, it may be considered possible 

 that the scale of symmetry suggested has a natural meaning, 

 and that the steps by which the symmetrical arrangement 

 has been acquired correspond to the degrees of symmetry 

 detailed above. For, it may be observed, that in no case did 

 I find other combinations, such as three tentacles in the upper 

 whorl and six in the lower, or three in the upper and five in 

 the lower, or five in the upper and three in the lower, etc. 



The variations appear to be sharp and definite, so that an 

 unit amount of variation, so to speak, is a discrete and quite 

 perceptible quantity. 



It is unfortunate that the scarcity of material prevents a 

 more thorough and exhaustive enquiry. 



